The semiconductor industry has, over the years, migrated to the use of copper for conductive traces, which metal offers lower electrical resistance, and thus signal impedance, than prior materials such as aluminum or aluminum alloys. This trend has been enhanced by the industry employing ever-higher frequency signals to accommodate faster switching speeds in electrical circuits, in order to maintain power consumption at a reasonable level.
One phenomenon associated with the use of copper traces, which is negligible at lower frequencies but which becomes significant at frequencies around 1 GHz and above, is the so-called “skin effect” responsive to the surface finish exhibited by the copper trace. As frequency increases, the skin effect drives the current into the surface of the copper, dramatically increasing power loss and reducing signal speed with increasing roughness of the surface finish. This is due to the effective length of the conductor increasing as the current follows along a rough surface topography of the copper. Thus, at high frequencies, the effective impedance of the copper increases as a function of the increased distance the current must traverse over the rough copper surface.
Conventional methods of fabricating conductive traces, such as for RDLs, involve depositing a blanket seed layer on a substrate, followed by depositing and patterning a photoresist, electroplating copper to form traces in the trenches in the photoresist, and then stripping the photoresist from the substrate to expose the seed layer, which is then etched.
The conventional fabrication process is illustrated in FIGS. 1A through 1E. In FIG. 1A, a substrate 100 has a seed layer 102 of a metal deposited, for example, as by physical vapor deposition (i.e., sputtering), to serve as an adhesion layer and as an electrode for subsequent electroplating of metal thereon. In FIG. 1B, a layer of photoresist 104 is deposited on seed layer 102, after which the photoresist is patterned, developed and portions of the photoresist 104 removed to form trenches 106. In FIG. 1C, copper is electroplated over the portions of seed layer 102 exposed in the trenches 106 to form conductive traces 108. The photoresist 104 is then removed, exposing conductive traces 108 which exhibit smooth surfaces S from the electroplating process as shown in FIG. 1D. However, as also shown in FIG. 1D, the portion of seed layer 102 previously covered by patterned photoresist 104 is now exposed, necessitating removal to avoid electrical shorting between adjacent conductive traces 108. When seed layer 102 is removed by wet etching as shown in FIG. 1E, the surfaces of the electroplated conductive traces 108 are also etched, resulting in rough surfaces R, increasing conductive trace impedance. As a result, high frequency signal transmission is impaired due to the skin effect, resulting in signal losses and requiring additional power to maintain signal speed.